From Lost Fables Wiki
Fairy sweat is exactly what its name would imply, and is thus extremely difficult to gather. Fortunately, it only requires 10mL for a dose, meaning that even the tiny amounts which can be gathered is, at least, enough for a few uses.
Fairy Sweat gathered in a small, specifically designed altar, three meters in diameter, which must be positioned in the moonlight. The altar itself draws fairies almost every night if milk and honey are left for them, who eat their fill, dance and make merry, sweat a little, and drop their sweat into the altar’s special grooves. At the end of the night, a partial dose will have formed. Twenty doses per age can be gathered this way.
The "mundane" use of fairy sweat is in 1mL per 100kg doses, frequently used in religious rituals across the world to cause intense, reality bending hallucinations, and spiritual connections with deific beings. The trip tends to last a very long time, and be at least partially nightmarish. When used for alchemical purposes, it wildly warps the normal effects of the ingredients already put into a mixture.